Explosives contained in ordnance may be exposed to fires aboard aircraft carriers in ammunition storage dumps and during transit. Detonation under such circumstance produces catastrophic results. Providing high explosives which merely burn after being heated in a fire has long been a goal in the explosive arts.
Explosives such as RDX (cyclotrimethylenetrinitramine) or HMX (cyclotetramethylenetetranitramine) have about 1.5 times the explosive power of TNT. However, they are too shock sensitive for use in the pure state and must be mixed with insensitive materials, which of course lower explosive power, for use in ordnance devices. A number of flexible, rubbery binder materials, including polyurethanes have been used with cyclic nitramine explosives (HMX and RDX) to form relatively insensitive and castable or moldable explosive compositions.
In the late sixties, the first elastomeric cast explosive was developed, using RDX and a polyurethane formed from polyethylene glycol and cured with toluene diisocyanate (TDI). This explosive was designated PBXN-106 and requires a plasticizer, the eutectic mixture of the formal and acetal of 2,2 dinitropropanol (BDNPF/A). This explosive is cook-off and bullet resistant but can only be made in limited quantities since BDNPF/A is no longer being made.
The possible unavailability of BDNPF/A prompted the Air Force in the 1970's to develop another castable explosive, PBXF-108, which contains a polyproplene glycol binder cured with three isocyanates: TDI, hexamethylene diisocyanate, and polyphenyl methylene isocyanate (PAPI). Again, about 82% RDX (desensitized by a coating of isodecyl pelargonate) is used in the form of class A 62% (150.mu.) and class B 20% (44.mu.).
Another plastic bonded explosive, PBXW-108, using a polyurethane binder was also developed in the early '70's. The binder is hydroxyl-terminated polybutadiene, cured with PAPI. Again the approximately 84% RDX must be coated with a plasticizer (dioctyl adipate) to insure safe handling and a rubbery binder.
Previous plastic bonded high explosives have a number of disadvantages. TDI is a very toxic curative. Polyethylene glycol is hygroscopic and must be handled under dry conditions because TDI and most other isocyanates react with moisture. The use of a plasticizer complicates processing, and plasticizers tend to migrate out of binders. Finally, some explosives, such as PBXF-108, require up to 4 particle sizes of RDX which must be separately prepared.